In the field of flight management systems (FMSs), the technical problem to be solved is related to the use by the ground of predictions calculated by the FMS along the flight plan (altitude, speed, fuel, time of passage, for each point on the flight plan). In recent studies, it emerged that a significant improvement in capacity and safety for future ATM (Air Traffic Management) systems lay on the one hand in the collaboration between the ground-based (ATC) and onboard (aircraft) operators, in particular the synchronization of route and flight data, and on the other hand in the deterministic predictability of the 4D situation of the traffic in the controlled air space.
The ground-based operators can use the predictions issued by aircraft to organize the traffic, smooth out the density of aircraft in each control sector, anticipate the dynamic control sector segmentations and groupings, sequence the aircraft more effectively in the terminal procedures, and lastly be able to deploy an end-to-end ATM system (“4D” and “Gate to Gate” concepts).
All these operations require both regular synchronization and precision in trajectory forecasts carried out on the ground and on board.
Current flight management systems (FMSs) provide for precisely calculating these predictions as long as the aircraft follows its flight plan. The problem lies in the quality of these predictions when the aircraft is no longer guided on the reference flight plan, in particular following a controller instruction urging it to temporarily leave its initial flight plan, and does not have instructions telling it where and when it can or must return to this initial flight plan. In this case, these are referred to as “tactical” instructions which are generally “open”, i.e. without limitations in space or time.                This takes place in particular when the controller has to manage two types of scenario:        a potential conflict with another aircraft,        ensuring longitudinal separation with the preceding or following aircraft by extending or shortening the trajectory of the aircraft in order to land.        
The controller then, using points, asks the aircraft to take a heading (most often the case, since this approach is the quickest to implement), or an altitude instruction, then returns the aircraft to its initial trajectory as soon as the nominal situation (separation) is restored. The controller can also issue a speed instruction for control purposes, but in very limited circumstances since this is not always possible (if the aeroplane is at the limits of its flight envelope), the execution takes time, is not very perceptible by the controller, and does not have a significant impact on the trajectory, and above all, this can cause delay in arrival.
In the vast majority of cases, the point-based trajectory modification is carried out within a single control sector, taking into account the LOA (Letter Of Agreement) which requires a sector A to transmit to an aircraft at a sector B at a given 3D point (position/altitude), and at a given speed.
Nevertheless, as long as the aircraft is held in “tactical mode”, the FMS no longer knows how to correctly calculate the point at which it rejoins its initial flight plan, since it does not know when the tactical instruction will stop and enable it to return to the initial trajectory. By default, the predictions calculated by the aircraft assume that it rejoins the initial route immediately. In the 4D concept, these predictions are not precise enough and can disrupt all the traffic. Under these conditions, it is not possible to increase air space capacities as required by future capacity plans.